1. Field of the Invention
The subject of the invention is a process for hollowing out a cavity, by dissolution, in a ground formation comprising at least one layer containing predominantly salt to be dissolved.
2. Description of the Background Art
The invention aims more particularly to obtain, after hollowing out, a subterranean cavity permitting the storage of a fluid and especially of natural gas in a stratified layer of salt, the thickness of which is typically of the order of several tens of meters, and more particularly between 30 meters and 100 meters. The salt may consist especially of sodium chloride (NaCl) or potassium chloride (KCl), without this being restrictive.
The object of the invention is in particular to provide a solution which will be low in cost in relation to the volume of the cavity. To this end, the invention tends to improve the shape of the cavity obtained in order to adapt it to the shape of the layer of salt and thus increase its volume.
However, since the cavity is to be subjected, in use, to the storage of gas, it is necessary to control the spread of the dissolution process in order to obtain a final cavity having a mechanically stable form. Moreover, the cavity obtained must provide a satisfactory seal with respect to the external environment.
U.S. Pat. No. 5,246,273 and WO-A-95 10689 do in fact describe a process comprising the following steps:
drilling at least one hole comprising a substantially horizontal section arranged at least in part in the salt layer, PA1 providing in the drilled hole(s) an injection pipe, an extraction pipe and a preliminary communication space connecting the injection pipe and the extraction pipe, PA1 injecting a solvent of the salt, into the communication space, through one end of the injection pipe forming an injection point, PA1 extracting, through the extraction pipe, the brine formed by the dissolution of the salt, in contact with the solvent. PA1 a plurality of preliminary sub-cavities in the salt layer, PA1 a succession of channels isolated from the said layer to be dissolved containing salt, the channels connecting the sub-cavities in a fluid manner two at a time in order to form an open circuit for circulation of the solvent, extending between a first sub-cavity and a last sub-cavity, the preliminary sub-cavities and the channels defining the said preliminary communication space. PA1 drilling the hole, PA1 introducing a casing into the drilled hole thus obtained, into the layer of salt, PA1 arranging a sealing material (such as concrete) between the casing and the drilled hole, and PA1 eliminating the casing and the sealing material in specific areas in order to form the preliminary sub-cavities.
However, the process described in WO-A-95 10689 is rather complex, since it requires the provision of different injection apertures, the dimensioning of which must be carried out accurately based on the evaluation of different parameters. Moreover, this solution raises the problem of reliability--behaviour over time--since the size of at least some of the apertures is likely to increase owing to wear in contact with the solvent and in other cases to decrease owing to the presence of insoluble elements in the salt layer.
The process described in U.S. Pat. No. 5,246,273 requires sophisticated equipment to control the injection of the solvent in a substantially horizontal plane. Moreover, the solution described in that document is not very well suited to cavities of several tens of meters in thickness and more than one hundred meters long. Finally, the cavity obtained does not guarantee mechanical stability and a satisfactory seal with respect to the external environment.
The document U.S. Pat. No. 3,510,167 discloses a process for improving the speed of dissolution of a layer of salt which consists in passing the solvent previously into another layer of salt in order to increase its density.
However, this process is remote from the invention. In fact, this process does not make it possible to control the shape of the cavity obtained. Furthermore, a plurality of separate cavities is obtained, and moreover these belong to separate layers of salt. The present invention itself aims to obtain a single cavity.